Since I'm unable to refer to the correct diagram, don't take these as gospel until they can be confirmed. Consider them as educated guesses until then. This is from an early Scout 800 diagram.
#15...b+ input from ammeter.
#40(2)..output to dash panel illumination.
#16...output to floor dimmer switch.
#20...output to front markers.
#22...output to stoplight switch.
#21...output to rear harness.
#118..output to flasher fuse via inline fuse.
So it seems this connector is for the headlight switch. It also looks original to the truck. The switch you purchased also "seems" to be correct for your truck, yet it does not appear to match the stock looking connector. Something's gotta give. You could clip all the wires to ditch the plug and make direct connections to the switch. It would be kind of a shame to have to do that. As long as I've been knocking around these forums, I don't recall anyone else having this issue. I also know of only two styles of headlight switches used on these rigs. The 60's era style such as you've purchased and the 70's era style, which has a much different looking connector than is under your dash. Unless there was a third style of switch that was also used. I'm kind of at a loss here. I guess if it was my truck and I was desperate to make this replacement switch work, I'd be snipping wires and getting rid of that stock plug. That has to be your call though. I'm not advising you to do it.